1. Field of the Invention
The present invention relates to zoom lenses and, more particularly, to a zoom lens having a diffractive optical element which is suited to lens-shutter cameras, video cameras, or like optical apparatuses.
2. Description of Related Art
For the lens-shutter camera or the like that does not require the long back focal distance, many zoom lenses so far proposed are of the type comprising, in order from an object side, a first lens unit of positive refractive power and a second lens unit of negative refractive power, wherein the air separation between these two lenses is varied to effect zooming.
Also, in Japanese Laid-Open Patent Applications No. Sho 56-128911, No. Sho 57-201213, No. Sho 60-170816, No. Sho 60-191216 (U.S. Pat. No. 4,659,186), No. Sho 62-56917, etc., zoom lenses which are improved to a compact form have been proposed each comprising, in order from an object side, a first lens unit of positive refractive power and a second lens unit of negative refractive power, wherein the separation between these two lens units is varied to effect zooming. As these zoom lenses employ the plus-minus refractive power arrangement in this order from the object side, the back focal distance is made relatively short. Moreover, the physical length for the telephoto end of the complete lens is shortened, while still maintaining realization of a high optical performance.
Further, as derived from the 2-unit zoom lens by dividing the first lens unit of positive refractive power into two parts of positive refractive powers, another type of zoom lens is obtained. That is, a zoom lens is constructed from three lens units in total and has a plus-plus-minus refractive power arrangement, whereby the action of varying the focal length is laid on the second and third lens units to assure a great increase of the zoom ratio. Such a 3-unit zoom lens has been proposed in, for example, Japanese Laid-Open Patent Applications No. Hei 3-282409, No. Hei 4-37810, No. Hei 4-76511, No. Hei 4-223419, No. Hei 5-264903, etc.
In the meantime, for the purpose of facilitating the correction of chromatic aberrations, it is known to provide part of an optical system with a diffractive optical element, as disclosed in Japanese Laid-Open Patent Applications No. Hei 4-213421 (U.S. Pat. No. 5,044,706), No. Hei 6-324262, No. Hei 9-19273 and No. Hei 9-197274.